1. Field of the Invention
The present invention relates to a mobile communication system and a transmission mode switching method used therefor as well as a recording medium having a program of the method recorded therein and, in particular, to a modulation/coding mode switching method in a system using an HS-PDSCH (High Speed-Physical Downlink Shared Channel).
2. Description of the Related Art
In recent years, a mobile terminal (mobile station) such as a cellular phone has been advanced to cope with multimedia for handling still images with a large amount of data, short animations or the like and a large-capacity and high-speed data transmission method is required accordingly.
As such a large-capacity and high-speed data transmission method, there are proposed a PDSCH system that makes only a transmission speed in downlink (direction form a base station to a mobile station) high, an HS-PDSCH (High Speed-Physical Downlink Shared Channel) system, or the like.
In a mobile communication system for carrying out data transmission from a base station to a mobile station using the above-described HS-PDSCH, any one of a plurality of modulation/coding modes such as a QPSK (Quadrature Phase Shift Keying) that is capable of transmitting two bits (four values) by one modulation, a 16QAM (16 Quadrature Amplitude Modulation) that is capable of transmitting four bits (sixteen values) by one modulation and a64QAM (64 Quadrature Amplitude Modulation) that is capable of transmitting six bits (sixty-four values) by one modulation.
An example of selecting one of the modulation/coding modes is shown in FIG. 40. In FIG. 40, for example, the modulation/coding mode is selected such that the closer a mobile station 102 is to a base station 101 in distance the faster the transmission is. That is, the base station 101 selects the modulation/coding modes of the64QAM, the 16QAM and the QPSK in the order of closeness to the mobile station 102.
As a method of selecting one of the above-described modulation/coding modes of the64QAM, the 16QAM and the QPSK, conventionally, there is a method of determining in advance a range of a receiving quality (Ec/Io (energy per one chip/interference wave power per unit frequency)) of a common pilot signal (CPICH: Common Pilot Channel) that is forwarded from a base station to a mobile station and uses respective modulation/coding modes and selecting a modulation/coding mode according to a receiving quality of a common pilot signal (hereinafter referred to as a first prior art).
In this case, the mobile station measures a receiving quality of a common pilot signal from the base station to inform the base station of the receiving quality. A modulation/coding mode is selected by the base station according to a receiving quality of a common pilot signal informed by the mobile station. For example, as shown in FIG. 41, the base station selects the modulation/coding modes of the 64QAM, the 16QAM and the QPSK in the order of excellence of a receiving quality of a common pilot signal informed by the mobile station.
In addition, as a method of selecting one of the modulation/coding modes of the 64QAM, the 16QAM and the QPSK, there is a method of detecting an error of HS-PDSCH data forwarded as a block from a base station to a mobile station and selecting a modulation/coding mode according to the block error rate.
In this case, the mobile station detects an error of an HS-PDSCH data block from the base station. The base station or the mobile station calculates a block error rate of the HS-PDSCH data at a predetermined period set in advance and selects a modulation/coding mode according to the block error rate.
For example, as shown in FIG. 42, the base station or the mobile station switches the modulation/coding mode to a low-speed mode if a calculated block error rate is larger than a predetermined block error rate T. In FIG. 42, it is switched from the 16QAM modulation/coding mode to the QPSK modulation/coding mode.
In addition, the base station or the mobile station switches the modulation/coding mode to a high-speed mode if a calculated block error rate is smaller than the predetermined block error rate T. In FIG. 42, it is switched from the QPSK modulation/coding mode to the 16QAM modulation/coding mode.
In the above-described conventional mobile communication system, in case of a first prior art, there is a problem that it is difficult to most appropriately set a range (threshold value) of a receiving quality of a common pilot signal corresponding to respective modulation/coding modes.
In addition, in case of the first prior art, since a measurement error is included in a receiving quality, it is difficult to select an optimal modulation/coding mode due to the measurement error as well. If measurement time is extended in order to make a measurement error of a receiving quality of a common pilot signal small, it is impossible to select a modulation/coding mode while following a change in conditions of a transmission path.
Here, as a factor for determining the conditions of the transmission path, there are a transmission loss, a multi-path environment (the number of paths and a size of each path), a noise power (interference wave power and thermal noise power), a moving speed of a mobile station, or the like. Thus, even if a receiving quality of a common pilot signal is identical, the above-described factor may be different, which makes an optimal mode of the HS-PDSCH different according to a multi-path environment or a moving speed of a mobile station. Further, the optimal mode means a mode with which a data transmission speed becomes maximum among modes that can satisfy a target communication quality (block error rate or the like).
Moreover, in the case in which a transmission power of the HS-PDSCH is changed while keeping a transmission power of a common pilot signal constant, if the mobile station performs mode selection, it is necessary for the base station to inform the mobile station of information on the change of the transmission power of the HS-PDSCH and the mobile station to select a mode based on the information. In this case, control information of which the base station informs the mobile station increases and an optimal mode cannot be selected while the base station sending the control information. To the contrary, if the base station performs mode selection, it is necessary for the base station to change a threshold value of each mode according to the change of the transmission power of the HS-PDSCH.
On the other hand, in a second prior art, it is necessary to extend measurement time of a block error rate in order to increase measurement accuracy of a block error rate, when a state of a transmission path changes, time required for changing a mode to an optimal mode becomes long. In particular, when an optimal mode turns into a low-speed mode, a state in which a block error rate exceeds a target value is likely to continue for a long time. In addition, if measurement time of a block error rate is reduced in order to solve this problem, a measurement error of a block error rate increases.
When a length of time when a predetermined block error rate is satisfied and a length of time when the predetermined block error rate is not satisfied are substantially equal in ratio, an entire block error rate may be larger than the predetermined block error rate. Thus, since it is necessary to set the predetermined block error rate larger than the target block error rate, optimization of the predetermined block error rate becomes difficult. Therefore, the second prior art also has a problem that selection of an optimal mode is difficult.